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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

POTABLE WATERS

il

9.  Lime.  The filtrate from the preceding operation is concentrated
to some extent and treated with ammonia and ammonium oxalate ;  the
calcium oxalate formed is filtered off, washed, dried, and strongly heated
in the blowpipe flame until it undergoes no further loss of weight ;  this
represents CaO.

10.  Magnesia.  The filtrate from the calcium oxalate is evaporated
to dryness in a platinum dish and the residue gently heated to expel all
the ammonium salts and then redissolved in very dilute hydrochloric acid.
The solution is neutralised with ammonia solution and treated with neutral
ammonium carbonate solution in such amount that the precipitate at first
formed redissolves.    The liquid is then left for 12 hours to allow of the
complete precipitation of the magnesium as magnesium ammonium car-
bonate, which is filtered off, washed with dilute ammonium carbonate
solution, dried, calcined, and weighed as magnesia (MgO).

11.  Alkalies.  The filtrate from the previous operation is evaporated
to dryness with ammonium chloride in a platinum dish, the residue being
carefully calcined and the pure alkali chlorides remaining then weighed.
If the chlorine is estimated, the amounts of K20 and NaaO present may
be calculated (see Fertilisers :   Stassfurt Salt).

12.  Poisonous and other Metals.  The residue from the evaporation
of some litres of the water may be tested for the poisonous heavy metals,
such as lead, copper, barium, etc., and for elements found only in traces
in potable waters, such as boron, iodine, bromine, lithium, etc.

13.  Calculation of the Analytical Results.  As a rule the amounts
of the different constituents are referred to i litre or 100 litres of the water,
all the elements estimated being expressed as oxides with the exception
of chlorine.

To check the results the amounts of the metallic oxides (Na20, CaO,
MgO) and of the acid anhydrides (S03, NaOG, Si02, C02) and chlorine are
added together, an amount of oxygen equivalent to the chlorine being
subtracted from the sum ; the remainder should be sensibly equal to the
fixed residue dried at 180.

The salts contained in the water may be reconstructed by uniting the
bases and acids in their most probable combinations. The chlorine is
first combined with the sodium and any excess (rare) with calcium. The
sulphuric acid is united with the lime and the nitric acid with the potash
and lime, or with the ammonia (if such is present). The rest of the lime,
magnesia and potash is united with the carbon dioxide ; the silica remains
free.

If, however, the evaporated water exhibits an alkaline reaction, it
contains sodium carbonate, generally together with sulphate and chloride ;
the lime and magnesia are then all combined with carbon dioxide.

A water may be said to be potable when, it is clear, colourless, odourless, of
pleasant taste, cool and of constant temperature and does not contain more
than certain limiting proportions of various dissolved matters. According to
different authors, these limits are as follows :t excess of ammonia